One of the widely used nondestructive inspection methods is ultrasonic flaw detection. In this method, however, a direct or an indirect contact between a probe and a material to be inspected through a contact medium is essential to obtain an acoustic connection between the material to be inspected and the probe for transmitting and receiving ultrasonic waves.
Ultrasonic flaw detection has been widely used as quality control means in steel material producing operations in steel mills. It has been recently attempted to perform flaw detection and scarfing conditioning of the flaw according to the results of the detection in steps as early as possible without passing any defective material to later steps, thereby increasing production efficiency and yield. For this reason, ultrasonic flaw detection of steel materials at high temperatures (800.degree. C.-1200.degree. C.) has been recently demanded.
However, the heretofore used common ultrasonic flaw detection methods requiring the direct or indirect contact between the probe and the hot steel material have many problems such as unduly large thermal affects from the hot steel material being inspected, thermal damage of the probe, deterioration of characteristics of the vibrator, and the like. In the flaw detection of, for example, steel materials at high temperatures (800.degree. C.-1200.degree. C.) during hot rolling, the heretofore used ultrasonic methods can produce little in the way of substantial results because a combustible material such as oil cannot be used as the contact medium, and other material such as water, when used as the contact medium, cannot provide a stable acoustic connection between the probe and the steel material due to rapid evaporation, and the probe itself is subjected to the very high temperatures.
As for scanning by means of a probe, the heretofore used ultrasonic flaw detection methods adopt either the system in which one probe reciprocates widthwise of the steel material or the system in which a plurality of probes are prearranged widthwise of the steel material to be inspected. Both the systems require large-scale equipment.
Non-contact ultransonic flaw detection methods have been developed to solve these problems. One of non-contact ultrasonic flaw detection methods, namely, electromagnetic ultrasonic flaw detection, will now be briefly described below.
When a static magnetic field is applied in a perpendicular or in a parallel direction with respect to the widthwise direction of a material to be inspected such as a steel material and a high frequency electric current is applied to a coil disposed opposite to the surface of the material to be inspected, ultrasonic waves are produced by Lorentz' force due to an eddy current induced on the surface of the material being inspected by the high frequency current and said static magnetic field. The ultrasonic waves are propagated through the material being inspected and are, when a defect is present, reflected or considerably attenuated by the defect before reaching the back surface of the material. In the absence of any defect, the waves are only slightly attenuated before reaching the back surface and are reflected thereby. In this way, since the ultrasonic waves reflected to the surface of the material being inspected or reaching to the back surface thereof can detect the eddy current generated by the ultrasonic vibrations and said static magnetic field applied to the front or the back surface of the material being inspected, by means of a coil disposed opposite to the front or the back surface thereof, it is possible to determine the presence of any defect and to locate it by analysis of the level and the time of reception of the ultrasonic waves, as in a pulse-echo or through-transmission method common in ultrasonic flaw detection methods using a transducer.
When the direction of the static magnetic field is parallel to the direction of movement of the surface of the material to be inspected, namely perpendicular to the widthwise direction of the surface of the material, transmission and reception of longitudinal waves is possible. On the other hand, when the direction of the static magnetic field is perpendicular to the direction of movement of the surface of the material to be inspected, namely parallel to the widthwise direction of the surface of the material, transmission and reception of transverse waves is possible.
In conventional electromagnetic ultrasonic flaw detection, since it is necessary to intensify the static magnetic field to increase the ultrasonic generation and reception levels, an electromagnet has usually been employed as the static magnetic field forming means and a very large amount of electric current is applied thereto. Such a probe requires water-cooling in order to remove the radiant heat from the material being inspected at a high temperature. The Joule heat generated in the electromagnet by passage of the large amount of current must also be removed to keep the electromagnet cooled. To cool the electromagnet it is necessary to arrange water passages as near as possible the coil of the magnet and, accordingly, to pay sufficient attention to the insulation system thereof. Therefore, the conventional electromagnetic ultrasonic flaw detection apparatus has a disadvantage that it requires a complicated cooling system, resulting in probes of large sizes. Further, when it has been necessary to use a plurality of probes of larger in size, the problem encountered has been that the density of inspection points has unavoidably become low. Generally, since the flaw detection has been performed in most cases with a plurality of probes, there has been difficulty that the source of electric current common to the electromagnets of the probes has had to be very large in scale.